This invention relates to indirect calorimeters for determining the metabolic rates of subjects by measuring their oxygen consumption during respiration over a period of time, and more particularly to such a calorimeter employing a flow meter and a capnometer to compute the difference between the inhaled gas volume and the volume of the exhaled gas less the exhaled CO2 volume.
Measurement of the energy expenditure of humans is important for a number of reasons, including the determination of the proper caloric content for feedings of hospitalized patients whose metabolisms may deviate from normal values, the monitoring of progress of weight loss diets to allow the adjustment of caloric inputs to achieve a target loss and the determination of energy expenditure during exercise.
A variety of indirect calorimeters for measuring oxygen consumption during respiration have been devised. One form of respiratory calorimeter, disclosed in my U.S. Pat. Nos. 4,917,108; 5,038,792; 5,179,985 and 5,178,155, measures the volume of a subject""s inhalations over a period of time, and the volume of the subject""s exhalations after carbon dioxide in the exhalations has been removed by an absorbent scrubber. These measurements are integrated over the time of measurement and the difference between the two summed volumes is a measure of the subject""s oxygen consumption. This follows from the fact that inhaled oxygen is either absorbed into the blood in the subject""s lungs or expelled during exhalation. Some portion of the blood absorbed oxygen is replaced with CO2. When the CO2 is removed from the exhaled volume, the summed difference between inhalation and exhalation volume over a period of time is equal to the absorbed oxygen.
In some versions of these prior calorimeters a capnometer was also used to measure the instantaneous value of the exhaled CO2 in a breath allowing the calculation of CO2 production, Resting Energy Expenditure (REE) and Respiratory Quotient (RQ).
The absorbent scrubber used with these previous systems, such as sodium hydroxide or calcium hydroxide, which reacts with the CO2 to form water plus a salt, has a limited ability to absorb CO2 and must be replenished after a period of use. The scrubber is also large and heavy relative to the other components of the calorimeter.
The present invention eliminates the need for the carbon dioxide scrubber used in my previous devices by measuring the volume of exhaled carbon dioxide and subtracting that volume from the total exhaled volume over the measurement period to calculate a sum which is then subtracted from the inhaled volume to arrive at VO2. The volume of exhaled carbon dioxide is preferably measured by integrating the instantaneous carbon dioxide percentage of the exhalation, as measured by a capnometer, over the exhaled volume as measured by a flow meter: VCO2=Ve(%CO2).
The flow meter generates an electrical signal as a function of the instantaneous flow volume and this signal is preferably sent to microprocessor-based computer along with the electrical output of a capnometer sensor. A preferred embodiment of the invention uses a bidirectional flow meter to measure both the inhaled and exhaled flow volume. A temperature and/or humidity conditioner may be utilized to equalize the temperature and/or humidity of the incoming air to that of the exhaled air so that uniform flow measurements may be made. Alternatively, the system could receive signals representing temperature, humidity and/or barometric pressure from sensors disposed in the calorimeter or externally, or keyboard entries and calculate correction factors for the flow measurement based on the signals. In this configuration the distinction between inhalations and exhalations is determined by the presence or absence of CO2 in the flowing gas is measured by the capnometer or by a zero crossing algorithm applied to the output of the flow meter.
Alternatively, the invention might employ a unidirectional flow sensor and conduits and one-way valves arranged so that both the inhaled flow volume and the exhaled flow volume pass through the flow meter in the same direction possibly providing a more precise flow measurement than the bidirectional flow sensor of the preferred embodiment.
The microprocessor, in addition to calculating and displaying the VO2, may calculate and display REE, RQ and the rate of carbon dioxide production.
Another alternative embodiment of my invention may be used to calculate the subject""s Cardiac Output implementing the noninvasive method of cardiac output measurement using partial CO2 rebreathing described in an article by Capek and Roy in IEEE Transactions and Biomedical Engineering, Vol. 35, pages 653-61, 1988. This embodiment of the invention employs a two stage measurement. In the first stage, the device is configured in essentially the same manner as the other embodiments of the invention to measure oxygen consumption. Over a period of use, such as three minutes, the microprocessor measures VO2, VCO2, and the end-tidal CO2 (etCO2) which is the carbon dioxide content of a breath at the end of an exhalation. These values are stored and the device is then switched to a configuration in which the end portion of each exhalation is not expelled from the device but is rather captured so that it forms the initial portion of the gas provided to the subject during the next inhalation. This is achieved by creating a dead space chamber in the exhalation passage. The subject breathes in this manner for a short period such as 30 seconds. During this period the breath-to-breath etCO2 and the total VCO2 are recorded. The computer then implements the calculation:       C    .    O    .    =            Δ              VCO        2                    Δ              etCO        2            
where xcex94VCO2 equals the difference in the total volume of exhaled CO2, per breath, during the two recordings and xcex94etCO2 is the difference in the end-tidal CO2 between the two recordings.